The invention relates to a method for the manufacture of galvannealed metal sheet which has been produced from IF steel. According to the conventional understanding, “galvannealed metal sheet” is understood as a hot-dip galvanised metal sheet marketed in the form of coils or blanks, which has been annealed after the hot dipping. The coating produced by this “galvannealing” process on the metal sheet base material usually only consists of iron-zinc compounds.
The term “IF (interstitial-free) steel” is understood as steels without interstitially dissolved alloying constituents which contain silicon and additional contents of titanium and/or niobium for the removal of the C and N atoms, in addition to other alloying constituents which may be required. Such steels are distinguished by good cold-formability as a consequence of a low yield point and are especially suited for the deep drawing of components.
Galvannealed metal sheet made of IF steel is used especially in the manufacture of automobile bodies. Here the highest requirements with respect to formability are imposed both on the base material and also on the coating applied thereto. Practice shows that with conventionally produced galvannealed metal sheet, increased abrasion occurs in the pressing tool. Regardless of the influences exerted by the specific forming conditions, this abrasion depends to a large extent on the steel composition and the conditions under which the steel has been produced. These production conditions directly influence the phase structure of the coating and thus the surface condition, homogeneity and strength with which the coating adheres to the base material.
Silicon contents of up to 0.1 wt. % are added to IF steels from which galvannealed metal sheet of the type under discussion is produced, to improve the adhesion of the zinc coating on the base material. As a result of the silicon alloying, a stronger grain-boundary occupancy is achieved. During forming these grain boundaries tear and form as such “preset breaking points” which prevent any further exfoliation of the coating.
The mechanical properties and with this, the forming behaviour of the base material are, however, worsened by the silicon alloying. For example, it was been established that the strength of the material deteriorates by respectively 1 N/mm2 when the silicon content is respectively increased by 0.01 wt. %.
Other investigations have shown that in the case of galvannealed metal sheet produced from IF steel having only low Si contents, for example, 0.012 wt. % and at the same time Fe contents in the coating layer between 7 wt. % and 12 wt. %, the coating only adheres poorly to the base material. At higher iron contents in the coating and higher Al contents in the galvanising bath, a toothed structure could be observed at the steel/coating layer interface through which the adhesion of the coating to the base metal is supported.
In practice, however, the adhesion of the coating to the base material cannot be improved either by increasing the Al content in the zinc bath or by increasing the fractions of Fe in the coating layer. This is because a high Al content in the zinc bath leads to a substantial alloying delay in the galvannealed reaction. This delay can only be compensated by increased furnace temperatures and longer furnace transit times. Both measures incur increased operating costs, reduced economic efficiency and greater wear on the furnace.
Also high Fe contents in the coating can only be produced by high galvannealing temperatures and/or long holding times. This has the consequence that the coating layer contains a clearly identifiable layer of gamma phases. This gamma phase layer then adheres to the base metal with increased strength. However, between the gamma phase layer and the relatively very much thicker delta phase layer located thereon, there is some weakening of the adhesive strength. As a result, the thick delta phase layer thus peels away under a corresponding loading so that the abrasion increases and the protection of the base material desired with the coating is also not ensured.
A method of the type specified initially is basically known, for example, from DE 198 22 156 A1. In the known method a hot strip is hot-rolled from IF steel, coiled and rolled into a cold strip. The cold strip is then recrystallisation-annealed in an annealing furnace, before it is finally provided with a zinc coating in a zinc bath.